1. Field of the Invention
The present invention relates to a resin composition for encapsulating light-emitting devices and to a light-emitting device module encapsulated with the resin composition.
2. Description of the Related Art
Light-emitting devices such as LEDs (Light-Emitting Diodes) have low power consumption, are small in size, and are lightweight, and light-emitting devices encapsulated with resin are used in various types of lamps. In recent years, both blue and white LEDs have been developed, and the luminance of LEDs has also increased. Accordingly, the applications of LEDs have been rapidly expanding and include forming a backlight light source for liquid crystal display panels, forming a light source for illumination, and forming signal lamps. Furthermore, the application of LEDs to the headlamps of automobiles is also being developed.
Traditionally, bisphenol A glycidyl ether type epoxy resins are used as an encapsulating resin for light-emitting devices such as LEDs. However, with the increase in luminance of LEDs, both the heat resistance and light resistance (particularly with regard to resistance to both UV and blue light) of such epoxy resins have become insufficient. Therefore, in high-intensity LEDs, UV LEDs, and the like, the encapsulating resin thereof is discolored by the heat or UV light emitted by these LEDs, and this causes a problem in that the luminance of LED modules reduces over time. In order to resolve this problem, highly transparent epoxy resins of improved type and the like have been developed. However, the heat resistance and light resistance of such resins are still not satisfactory. In addition to this, since epoxy resins become hard once cured, a problem arises in that thermal stress may, for example, cause breakages in the wiring or peeling at the junction between an electrode and the wiring in LED modules.
In view of the above, gel-type silicone resins have recently been used in high-intensity type LEDs as they exhibit excellent heat resistance and light resistance, and also exhibit excellent physical flexibility. However, the gel-type silicone resins are two-part type curable resins in which the hydrosilylation reaction is utilized, and hence the gel-type silicone resins have to be used within their pot-life after mixing.
Moreover, since the refractive index of the gel-type silicone resins is lower than that of epoxy resins, the silicone resins have a drawback in that they reduce the light extraction efficiency of the LED modules that include these resins. Specifically, in the most common types of high intensity LED modules, an LED chip is placed face down on a sapphire substrate having a high refractive index (being 1.76), and light is extracted from the sapphire substrate side of the LED module. In order to extract light from the sapphire substrate into an encapsulating resin efficiently, it is preferable that the refractive index of the encapsulating resin be close to the refractive index of sapphire, i.e. 1.76. However, among the various silicone resins used as an encapsulating resin, dimethyl silicone resin has a refractive index of 1.41. In addition to this, diphenyl dimethyl-based and phenyl methyl-based silicone resins, into which a phenyl group is introduced to increase the refractive index thereof, only have a refractive index of approximately 1.51. When an excessively large number of phenyl groups are introduced to a silicone resin in order to increase its refractive index, the viscosity increases too much and the silicon resin therefore becomes unsuitable for pouring into a mold, and the physical flexibility of the cured product of the resultant resin is therefore impaired. Hence, the refractive index of silicone resins used as an encapsulating resin is lower than that of epoxy resins, which have a refractive index in the range of 1.53 to 1.57. Therefore, at present, while silicone resins are used, there is a compromise made with regard to light extraction efficiency in order to still take advantage of their excellent heat resistance, light resistance, and physical flexibility.
Meanwhile, there is an attempt to increase the refractive index of a resin as a whole by adding fine particles having a high refractive index to the resin (see Japanese Patent Application Laid-Open No. 2004-15063). In this case, titanium oxide, zirconium oxide, zinc oxide, and the like are considered to be used as the high refractive index fine particles. However, in order to increase the refractive index of the silicone resins to a desired level using this technique, a relatively high volume percent of, i.e., at least 10 to 40% by volume of the fine particles must be mixed into the silicone resin being used. At such a high mixing ratio, high transparency cannot be obtained, and furthermore, appropriate fluidity cannot be obtained at the time of pouring the silicone resin into a mold. Hence, there has been an attempt to improve the transparency of silicone resins using fine particles called single-nano size particles. However, as the cohesive force of the ultrafine particles of single-nano size is very strong, it is very difficult to uniformly disperse the ultrafine particles within a resin without forming secondary aggregated particles. Therefore, a technology for increasing the refractive index by adding such fine particles has not yet been practically realized. Accordingly, in high-intensity LED modules, a gel-type silicone resin alone is still often used even though the refractive index is compromised.